Because of their lightweight, high strength and rigidity, fiber-reinforced plastics are widely used as fiber-reinforced composite materials in products ranging from sports/leisure applications to industrial applications such as automobiles and aircraft.
Among fiber-reinforced plastics, fiber-reinforced plastic tubes are especially widely used in sports/leisure applications such as fishing rods, golf club shafts, ski poles, bicycle frames and so on. In recent years, improving the fracture toughness of fiber-reinforced plastics in sports/leisure applications has been required to enhance feel during usage or feel when hitting a ball.
A method for producing fiber-reinforced plastics is, for example, using intermediate material (prepreg) formed by impregnating a matrix resin into reinforcing material made of reinforcing fibers such as long fibers. Such a method is useful because the content of reinforcing fibers is easier to control in a fiber-reinforced plastic and a plastic is designed to have a greater amount of reinforcing fibers.
Specific examples of a method for producing a fiber-reinforced plastic from a prepreg are molding using an autoclave, compression molding, internal-pressurizing molding, oven molding and the like. To cure a generic prepreg using such a method, it is usually necessary to thermoset the prepreg by applying heat for approximately two hours. Considering the time for heating or cooling the equipment, a molding die or the like to be used for curing, it takes approximately 2˜6 hours, depending on conditions, to cure a prepreg in one molding process. Accordingly, time taken for thermosetting is a factor that increases the cost of molding. In addition, a method has been sought for molding a product at a lower temperature in a shorter period of time so as to achieve mass production of a product.
As a method for shortening the molding time of fiber-reinforced plastics, it is an option to use a highly reactive epoxy-resin composition as a matrix resin so that curing reactions progress by a slight amount of energy, thereby shortening the time for curing the epoxy resin composition. However, if a resin composition is too highly reactive, curing reactions progress even when the composition is stored at room temperature, causing a decrease in the storage stability of the resin composition. Also, if the matrix resin composition cures too quickly, that may cause voids in a molded product produced using a method for molding in a short period of time. Especially, voids tend to remain in a molded product when oven molding (vacuum bag molding) is employed. Voids may be suppressed by lowering the viscosity of the matrix resin composition. However, a prepreg containing a low viscous matrix resin composition tends to be tacky at room temperature and is hard to handle during a production process.
Considering the problems above, epoxy-resin compositions are required to have the following features: properties for producing a molded product at low temperature in a short period of time; processability of a prepreg at room temperature and suppression of voids in a molded product; and excellent mechanical characteristics, especially excellent fracture toughness, when used as a matrix resin to produce fiber-reinforced plastics.
As an example of a prepreg capable of producing molded products at a relatively low temperature in a short period of time, patent publication 1 discloses a prepreg containing an epoxy-resin composition as a matrix resin, where dicyandiamide is used as a latent curing agent, and polyvinyl formal is used as a thermoplastic resin elastomer. Also, patent publication 2 discloses a prepreg formed by using an epoxy-resin composition containing a reaction product of an epoxy resin and an amine compound having a sulfur atom in the molecule.
In addition, to improve fraction toughness of a cured product, numerous methods are proposed using epoxy-resin compositions containing thermoplastic resins. For example, patent publications 3 and 4 each propose to use an epoxy-resin composition with an added polyamide-based thermoplastic elastomer. Also, patent publications 5 and 6 each propose to use an epoxy-resin composition with an added block copolymer.